How long does it take to write a Brainfuck interpreter? A flash coding exercise.
I'm afraid I did underestimate the effort, and it was not twenty, but rather thirty minutes (if one includes source control bookkeeping, and debugging, but still does not count creating proper test harnesses ;).
v1.0 was written on 2017-Jan-23, 18:33 to 19:03 UTC.
v2.0 was written in the following half an hour after feed-back from @Garmelon (more exactly, his implementation).
v3.0 adds a minor (or major?) enhancement in that custom commands can be actually specified.